In recent years, as the automation of control using an electronic control device configured by the use of a microcomputer or the like progresses, there has been an increasing need of the safety and reliability of the electronic control device. For example, in the event of the occurrence of an abnormality in an electronic control device, it is needed to immediately stop the operation or to continue the control operation to the possible extent while the safety is ensured.
As an example of performing control using an electronic control device such as the one described above, a vehicular electrically driven power steering apparatus is known. With an improvement in performance in recent years, electrically driven power steering apparatuses are also applied to a large-sized car with a great weight. However, particularly, a large-sized car requires a large human-powered steering force when the operation of an electrically driven power steering apparatus is stopped. Therefore, it is necessary to continue the operation of the electrically driven power steering apparatus even in the event of the occurrence of a malfunction while the safety is ensured.
With an improvement in degree of integration of a microcomputer in recent years, there is a concern about an increase in rate of occurrence of a soft error due to disturbance, e.g., cosmic rays. Soft error is also referred to as SEU (Single Event Upset) and is categorized as a temporary failure (transient fault) that occurs during the operation of a microcomputer. When a soft error occurs, a bit inversion occurs in data stored in the microcomputer, resulting in generation of abnormal data. In this case, in the case of data stored in a memory of the microcomputer, the abnormal data can be detected or corrected by means of an error correcting code (ECC). However, generally, an error correcting code is not added to data stored in a register in a processor of the microcomputer. Therefore, the occurrence of an abnormality in the data of the register due to a soft error can lead to false operation or runaway of the processor. Furthermore, in this case, the data stored in the memory can be lost due to false operation or runaway of the processor. As a result, safe control becomes difficult to continue.
It is preferable that an electronic control device safely continues control even in the event of the occurrence of a data abnormality due to a soft error such as the one described above. In this respect, PTL 1 described below discloses an electrically driven power steering apparatus for performing feedback control on the basis of a motor current detected by a current detector wherein, when feedback control is disabled due to a malfunction of the current detector, switching to feedforward control that does not use a motor current detected by the current detector is performed and assisting of handle steering is continued.